UPDATE 12/15/2008: I committed a change yesterday that changes the behavior of the .with method to use DELEGATE_FIRST as the resolveStrategy instead of OWNER_FIRST. If you are not sure what that means, you should by the end of this post.

Strange enough title.

Let's start with a hypothetical conversation between a geeky developer and his much less geeky wife:

Jeff: Betsy, how are you?

Betsy: I am fine thanks. How are you?

Jeff: Betsy, I am fine thank you.

Betsy: Great.

Jeff: Betsy, you know my birthday is the day after tomorrow right?

Betsy: Yes, I haven't forgotten. You mention it about 9 times a day you know.

Jeff: Betsy, yes I know. Are we going to have an ice cream cake?

Betsy: Yes, I think that would be good.

Jeff: Betsy, are you going to buy me the new Opeth DVD?

Betsy: I will get it for you but that music sucks big time.

Jeff: Betsy, that is awesome. Thank you.

Betsy: Why do you keep saying "Betsy" at the beginning of every sentence?

Jeff: Betsy, I guess I am used to inflexible languages which aren't very expressive.

Um, what does any of that have to do with Groovy? Well, lets talk about the problem with this conversation (aside from the lady's lack of appreciation for Swedish heavy metal). What is wrong is Jeff begins each sentence with "Betsy". Why might he do that? One reason is so Betsy knows that he is talking to her. Clearly this isn't necessary. It isn't necessary because she already knows he is talking to her. A context has been established which makes the addressing unnecessary. Jeff began the conversation by addressing Betsy, they probably made eye contact and were in close proximity. Once the conversation started, there isn't much question about who each sentence is being directed to.

Groovy developers can look at that and find quite a bit of noise that doesn't really have anything to do with what the code is trying to do but I want to focus on one specific thing. That one thing is all of the interaction with the calendar variable. Notice that we call clear() on the calendar, then call set() several times and later call getTime() on that same variable. All of those calls are prefixed with "calendar." so the compiler knows what we want to do. If we called "clear()" instead of "calendar.clear()", what would that mean? Are we calling the clear() method in this class? If that was our intent, it would not work because there is no clear() method. We have to prefix the call with an object reference so the compiler knows where to send the request. That seems to make sense. However, if we are going to do a bunch of things "with" the same object, wouldn't it be kind of nice if we could somehow do all of those things without all the repetition. Specifically, it might be nice if we could get rid of all of those "calendar." prefixes.

On to Groovy...

The following Groovy code does the same thing that the Java code above does.

Wow. That is a good bit cleaner than what we started with. Part of the reason for that is we were able to get rid of all of those "calendar." prefixes. What allowed us to do that is calling the "with" method on the calendar object and passing a closure as an argument. What we have done there is establish a context that says "do all of this stuff with this calendar object". When the closure executes, the calendar is given an opportunity to respond to method calls like clear() and set() and the implicit call to getTime() when referring to the "time" property which is being passed to println. Likewise, the references to MONTH, JULY, DATE and YEAR properties are also being handled by the calendar object.

That is pretty slick. Lets dig just a little deeper to get a little better understanding of what is really going on.

Groovy closures have a delegate associated with them. The delegate is given an opportunity to respond to method calls which happen inside of the closure. Here is a simple example:

// assign a delegate to the closuredef sb = new StringBuffer()myClosure.delegate = sb

// execute the closuremyClosure()

assert 'Jeff was here.' == sb.toString()

When the closure is executed, those calls to append() in the closure end up being sent to the delegate, the StringBuffer in this case.

Something similar is happening in the Groovy calendar code above. A closure is being passed to the with() method. That closure is calling methods like set() and getTime() which don't really exist in that context. The reason those calls don't fail is the with() method is assigning a delegate to the closure before it is executed. The delegate being assigned is the object that the with() method was invoked on. In the calendar example, the delegate is the calendar object. Something like this is happening...

This code does the same thing as the first Groovy example. Obviously the first one is cleaner.

I sort of lied a bit, or at least left out a detail that may be significant. The closure that is being passed to the with() method is really being cloned and it is the clone that is assigned the delegate and executed. This is a safer approach than monkeying with the original closure. If the reasons for that aren't clear, the explanation is another story.

Another bit of info that is missing here is the strategy that a closure uses to decide when to send method calls to the delegate. Each Groovy closure has a resolveStrategy associated with it. This property determines how/if the delegate comes in to play. The 4 possible values for the resolveStrategy are OWNER_FIRST, DELEGATE_FIRST, OWNER_ONLY and DELEGATE_ONLY (all constants defined in groovy.lang.Closure). The default is OWNER_FIRST. Consider the owner to be the "this" wherever the closure is defined. Here is a simple example...

// the append method on buffer will // be called because the delegate gets // first crack at the call to append() closure() }

static void main(String[] a) { new ResolutionTest().doIt() }}

So you see how the with() method helps establish a context where a bunch of things may happen "with" a specific object without having to refer to that object over and over. This can help clean up code like the Java code we started with. This approach can also be really useful when building a domain specific language in Groovy, a topic for another time.

I find this kind of programming very intuitive. Ruby 1.9 introduces a new method to the Object class called instance_exec which allows you to do this same kind of thing. I abhor typing so I think with is a better method name than instance_exec. *sigh*

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That is a nice example, also gave me a better understanding of Closures. I admit I have been just using them without understanding much. I know I have dealt with this in Java by using return values, if the method (particularly a setter) returns void, have it return the object it refers to. Makes the code cleaner, but will not help with the other 99.99999% of Java code that has already been written. Sigh makes me wish to use Groovy in my development work more. Need to work on getting Eclipse PDE working better with Groovy so that I can.

Jeff, I thought your post was going to solve my dilemma, but it didn't help. I'm trying to figure out how to access "this" at evaluation time, not closure creation time.

For example, I create a dynamic getter and setter by adding them to the metaClass. But when the getter or setter is evaluated, I can't access fields of the object whose property is being accessed, because "this" was locked down when the closure was first created.

If this doesn't make sense, there's a thread named Combining evaluate with MOP on the groovy-user mailing list where I've posted broken code. :-)

I sure could use a solution to this. I'm confident I'm doing something wrong. I just don't know how to fix it.

hahah the conversation kills me xD!It's like a conversation between, php and oracle... who can understand that, because like the guy there's not point in saying a name at the begining of every sentence ? now there's any kind of logic of doing it ? what you think ?

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@Marc Palmer: Thank you! I thought I was going crazy, because I was certain that there was a with token in Java, but I must have been remembering my Pascal/Delphi days! I remember spending $55 at the University bookstore for a copy of Turbo Pascal and developing console applications. I didn't have the $80 for Turbo Pascal for Windows, which came out within a few weeks of me buying my copy of Turbo Pascal. LOL

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